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Comment on “QA procedures in radiation therapy are outdated and negatively impact the reduction of errors” [Med. Phys. 38, 5835 (2011)]

2012; Wiley; Volume: 39; Issue: 2 Linguagem: Inglês

10.1118/1.3679336

2473-4209

Fritz Hager,

Radiation Dose and Imaging

To the Editor, In the November 2011 Point/Counter Point debate, both authors referred to the incident in Tyler TX as events where “QA procedures were not followed” or where “horrific medical physics errors occurred.”1 As the physicist of record in Tyler at the time of the Therac 25 Malfunction 54, I would like to clarify the record regarding this event. In contrast to the statements made by the authors, Malfunction 54 was not the result of a medical physicist error or failure to perform QA. The incident was the result of a well documented software error that permitted theTherac 25 linac to operate in photon mode without a target or flattening filter in the beam, resulting in a near-instantaneous dose of ∼16 000 cGy. There was no warning for the therapist and very little information from the Therac 25 that it had delivered such a high dose; the Malfunction 54 code was a general indicator that either too high or too low a dose had been delivered. As one of the first digitally operated linacs, spurious malfunction codes occurred periodically; the manufacturer [Atomic Energy of Canada Ltd (AECL)] indicated these were due to machine noise and that there were no actual problems. Following the first event, three AECL engineers examined the Therac 25 and could find no problems. At the time of the second incident, the therapist was teaching a student and recalled changing the mode on the Therac 25 from photons to electrons. With her recall of the keystrokes used for the patient, we were able to reproduce the Malfunction 54. These instructions were subsequently used to reproduce the Malfunction 54 type event at all Therac 25 locations. There was a similar Therac 25 error in Yakima, Washington resulting from a different series of events—toggling from light field to treatment mode caused the target to be left out—that also resulted in a patient death. The Therac 25 errors have been described in detail,2 were the result of manufacturer software deficiencies, and had nothing to do with the lack of a qualified medical physicist or attention to QA. Additional details on the Therac 25 and other linac accidents will be described in an upcoming review article for Medical Physics.